A PON system includes a configuration in which an OLT (Optical Line Terminal) and each subscriber's ONU (Optical network Unit) are connected with optical transmission lines made exclusively from passive optical elements. In other words, a 1:n optical splitter is disposed between an optical fiber connected to the OLT and a plurality of optical fibers respectively connected to the ONUs.
In an E-PON (IEEE 802.3ah) system, which has been standardized in July, 2004, a logical link can be established on an optical transmission line using an LLID (Logical Link Identifier) disposed on a preamble part of an MAC frame. Although all users share the same optical transmission line in the E-PON system, communications of each user are logically separated per link and therefore the system is like an aggregate of point-to-point connections. It is possible to establish a plurality of links with a single ONU and in this case, since SLA (Service Level Agreement) can be established per LLID, there is a merit to decide detailed bandwidth settings such as the minimum bandwidth assurance and the maximum allowable bandwidth for each application.
In the E-PON system, a MAC layer is extended to define a layer called multipoint MAC layer and a control protocol, MPCP (Multi-point Control Protocol), is provided to transfer a MAC frame. The basic functions of the MPCP are (1) Gate process to inform transmission timing to an ONU, (2) Discovery process to newly register the ONU under the OLT, (3) Ranging Timing process to measure a distance to the ONU to adjust the timing, and (4) Report process to demand a communication from the ONU to the OLT. The details of the Discovery process are described in IEEE 802.3ah, Draft 3.3, Clause 64.
To detect a newly participating ONU, the OLT regularly launches the Discovery process. In this Discovery process, the OLT first broadcasts upstream-transmission permitting frames called Discovery_Gate to the whole ONUs. A local time T1 of the OLT is recorded in the Discovery_Gate. An ONU that is newly powered on, namely an ONU that is not registered to the OLT, sets its clock according to the local time T1 of the OLT included in the Discovery_Gate, determines a transmission time T2 to the OLT using random numbers, and transmits a Register_Request signal to the OLT to request its register with the OLT at the time T2 together with the information of the time T2. The OLT calculates a distance to the ONU based on the time T1, Time T2, and a time T3 at which the OLT received the Register_Request signal. The OLT transmits to the ONU a registration notification signal, Register, on which an LLID is written. The OLT also determines the transmission time and the period to be taken for the transmission based on the distance to the ONU and informs the information to the ONU by a Gate message. The ONU transmits Register_ACK signal, on which the informed LLID is written, to the OLT at the time indicated on the Gate message. With the above operations, the ONU that was newly powered on is registered with the OLT and establishes a logical link with the OLT.
A plurality of logical links can be established with a single ONU. For instance, in a case that a logical link is to be established per application, when a logical link is established separately with each of three applications, VoIP, VoD (Video on Demand), and the Internet, detailed settings such as the minimum bandwidth assurance and the maximum allowable bandwidth can be made for each application. In this case, however, every ONU has to establish the whole three logical links when it is powered on. Thus, the processes required for establishing logical links also become three times. Therefore, when a number of ONUs are powered on at the same time, it takes a quite long time until the whole ONUs complete the establishment of logical links.
In the E-PON system, upstream signals are access-controlled through dynamic bandwidth allocation. That is, the OLT informs each ONU of a transmittable bandwidth and transmission timing through a Gate message and then each ONU transmits an upstream signal to the OLT using the transmittable timeslot informed by the Gate message. Each ONU further transmits a Report message in the end of the transmittable timeslot to receive a next Gate message. Each ONU informs the OLT of a volume of frames to be transmitted next and requests a bandwidth available through the Report message. The OLT collects the requests for bandwidths from the ONUs, determines bandwidths to be assigned to the ONU according to the DBA algorism, and transmits a Gate message indicating an assigned bandwidth together with a downstream signal to each ONU.
Even if no communications are performed, as far as an ONU is being powered on, the OLT and each ONU exchange DBA control messages of Gate and Report in order to maintain the links. This exchange of the DBA control messages is performed per LLID.
As explained above, in conventional methods, it takes really long time for an ONU to become available because the ONU tries to establish the whole logical links to be used at once when the ONU is powered on.
In addition, when a plurality of LLIDs, e.g. three logical links are assigned to a single ONU, the ONU needs a Discovery process and DBA control processes of Gate/Report for each logical link. In the three logical links, if there is a logical link that is not used, the DBA process performed for the nonuse logical link becomes useless and waste of limited transmission bandwidths. Due to this useless DBA process, a load of the OLT is also increased.
The above problems can be solved by dynamically establishing logical links.